Postsynaptic currents at the Mauthner fiber giant synapse of the hatchetfish

Choline acetyltransferase immunohistochemical staining in the goldfish (Carassius auratus) brain: evidence that the Mauthner cell does not contain choline acetyltransferase

In the hatchetfish, the Mauthner cell (M-cell) is thought to be cholinergic based on electrophysiological studies using cholinergic agents and on the localization of acetylcholinesterase (AChE) and alpha-bungarotoxin to M-cell-giant fiber synapses. Immunocytochemical studies have shown that mammalian and non-mammalian cholinergic neurons stain positive for choline acetyltransferase (ChAT), the enzyme responsible for synthesizing acetylcholine. We processed tissue from the goldfish (Carassius auratus) for the immunohistochemical detection of ChAT using the monoclonal antibody AB8 and the peroxidase-antiperoxidase procedure. ChAT immunoreactivity was found in selected areas of the goldfish brain including the cranial nerve nuclei and the ventral horn motoneurons of the spinal cord. Interestingly, the M-cell soma which stains positive for AChE was ChAT negative. This immunohistochemical evidence does not support cholinergic functioning of the Mauthner cell.

Ultrastructure of the rectifying electrotonic synapses between giant fibres and pectoral fin adductor motor neurons in the hatchetfish

Synapses formed by giant fibres on pectoral fin adductor motor neurons were identified by horseradish peroxidase (HRP) injection. The synapses were distributed in clusters on the somata and proximal dendrites of the motor neurons. All of the labelled synapses contained synaptic vesicles and often had clearly defined active zones characteristic of chemical synapses. Some synapses also showed gap junctions with the motor neuron soma, often directly adjacent to an active zone. The gap junctions were asymmetrical, with a thick layer of electron dense material on the postsynaptic side. Previous electrophysiological data indicate that giant fibre inputs to motor neurons are purely electrotonic and that these electrical synapses rectify.

Time course of miniature postsynaptic potentials at the Mauthner fiber giant synapse of the hatchetfish

The hatchetfish Mauthner fiber is presynaptic to 8-14 large myelinated axons in the medulla; the large ('giant') synapses formed by these fibers appear to be nicotinic cholinergic. Miniature postsynaptic potentials (mPSPs) were recorded from single identified synapses. The mPSPs were averaged to more accurately determine their shape; the rise time was approximately 70 microseconds, and the fall usually was biphasic with time constants of decay for the two phases of 280 and 800 microseconds. In 25% of the records analyzed a third, slow tail of decay was seen which had an average decay constant of 4.2 ms. The biphasic decay of mPSPs largely accounts for the similar shape of the postsynaptic current following a presynaptic impulse, which is described in the accompanying paper.